1. Field of the Invention
The present invention relates to a mechanical sensor for sensing a sudden deceleration of a vehicle.
2. Background Information
Among known vehicle seat belt units there are those equipped with a so-called pretensioner. At the time of a sudden acceleration (including deceleration) of the vehicle, the pretensioner retracts the webbing which a passenger is wearing by a predetermined amount so as to forcefully take up slack in the webbing thereby improving the restraint of the passenger.
In pretensioners of this type the tautness of the webbing is caused by the forced rotation of the webbing retractor shaft, while other types cause the tension of the webbing by a forced pull of the buckle. A pretensioner of the type that pulls the buckle to tension the webbing, for example, is equipped with a gas generator having a mechanical igniting sensor. The gas generator, which is provided with a cylinder, is connected to a buckle via a wire or the like.
At a sudden deceleration of a vehicle, the mechanical igniting sensor senses the deceleration so as to activate the gas generator. The instantaneous generation of gas moves the cylinder so that the force of the movement is transmitted to the buckle via the wire so as to tension the webbing.
The mechanical igniting sensor employed in a pretensioner of this prior art is basically composed of an ignition pin that fires a detonator, an inertial body that is moved inertially due to a large acceleration, a trigger member such as a ball that is present between the ignition pin and the inertial body so as to restrain the ignition pin from moving. One example is seen in U.S. Pat. No. 3,638,501. In this mechanical igniting sensor, in which only the above-mentioned trigger member is provided between the inertial body and the ignition pin so as to prevent the ignition pin from moving out of its normal state, there are many factors which affect precision(the sensitivity of the operation of the mechanical igniting sensor) during operation from the inertial movement of the inertial body that releases the hold on the ignition pin to the subsequent firing of the detonator. For example, there are directional differences between the pressure that the ignition pin applies to the trigger member and the pressure that the trigger member applies to the inertial body. Furthermore, these pressure directions change with the movement of the inertial body. There are other elements that affect the sensitivity of the operation of the mechanical igniting sensors. For example, the frictional force between the trigger member and the inertial body changes greatly during the movement of the inertial body. Because of these factors, the sensitivity of the operation is not steady, and a stable sensor operation is difficult to obtain. In some cases, the pressure on the trigger member coming from the ignition pin can be amplified because of the change in the direction of the pressure. In such cases, the amplified pressure given to the inertial body increases the frictional force between the trigger member and the inertial body. As a result, it causes the dispersion of sensitivity and the loss of operation stability.
In addition, the pressing forces on the trigger member of the mechanical igniting sensor in the prior art tend to counteract each other. It is therefore possible that the trigger member will fail to move as it should or stall even under a predetermined inertial force.
In this case, to simply increase the inertial mass so as to decrease the effects of the frictional force between the trigger member and the inertial body would only increase the size and weight of the unit as a whole.
Furthermore, in the operation of the mechanical igniting sensor in the prior art, once the trigger member has come out from between the inertial body and the ignition pin and the firing pin has been released from constraint, it is extremely difficult to reset the sensor to a state in which the movement of the ignition pin is prevented.
To test the mechanical igniting sensor, it is possible to test the sensor with respect to sensitivity and other operational properties and confirm the exactness of the operation. However, it is not possible to combine the tested sensor with a gas generator so that the unit will become operational reset. A solution to this problem, therefore, is needed for it is not possible to Omit the test operation of the mechanical igniting sensor.